1. Field of the Invention
The invention relates to image intensifier tubes of the type wherein, firstly, an incident ionizing radiation is converted into photons in the visible or near-visible range and, secondly, an array comprising microchannels is used to achieve a gain in electrons.
2. Description of the Prior Art
Image intensifier tubes such as these are commonly used in the fields of radiology, and especially in X-ray diagnosis, where they are called X-ray or radiological image intensifier tubes (or also "IIR" tubes).
The principle of a radiological or X-ray image intensifier tube is well known. It is illustrated schematically in FIG. 1, by a sectional view of an X-ray image intensifier tube 1.
The X-ray image intensifier tube 1 comprises a vacuum-tight chamber, constituted by a central body 2 with a shape generated by revolution positioned about a longitudinal axis 3. The body 2 is closed at one end by an input window 4 and at the other end by an output port 5.
Incident X-rays penetrate the X-ray image intensifier tube through the input window 4 which, for this purpose, should be as transparent as possible to these rays: the window 4 is generally constituted by a thin metal foil of aluminium or tantalum or glass etc.). An appropriate shape and appropriate mechanical characteristics give the window 4 mechanical resistance that is sufficient to withstand the atmospheric pressure exerted from the exterior to the interior of the tube.
The X-rays then encounter a set called a primary screen 15 which converts the incident X-radiation into electrons that are sent out into the vacuum from the point at which this radiation is absorbed. The primary screen is generally constituted by a "sandwich" which successively comprises: a support 6 that is transparent to X-rays, a layer 7 of scintillating material that converts the X-ray radiation into radiation of lower energy, generally in the form of visible light, and a photocathode 8, deposited on the scintillator 7, that sends electrons out into the vacuum under the effect of the radiation emitted by the scintillator.
The scintillator support 6 should be transparent to the X-rays: it is generally constituted by a thin sheet of metal or silica-based glass, etc.
The scintillator 7 is often constituted by layer of caesium iodide having a thickness of the order of 0.2 to 0.8 mm.
The photocathode 8 is formed by a layer of a photoemissive material generally having a very small thickness (often smaller than one micrometer).
The X-ray image intensifier tube 1 further includes a set or system of electrodes 10 set to potentials (not shown) suited to the accelerating and focusing of all the electrons emitted by a same point of the photocathode 8 on a homologous point of a luminescent screen 11 located on the output port 5 side. This system of electrodes is called the electronic optical system of the X-ray image intensifier tube 1.
The luminescent screen 11 is formed by a layer deposited on a transparent support 12, located inside the tube and behind the output port 5. It is thus possible, through the output port, to observe the converted visible image of the X-ray image which has been projected on the primary screen 15 through the input window 4 of the tube.
In an X-ray image intensifier such as this each incident X photon of primary energy ranging from 30 to 100 kV, absorbed in the scintillator 7, typically gives rise to several thousands of light photons and, thereby, to the emission of several hundreds of electrons into the vacuum, the quantum yield of the photocathodes 8 generally ranging from 10% to 20%.
Each of these electrons, accelerated at a voltage of 10 to 30 kV, in turn prompts the sending out of several hundreds of light photons in bombarding the luminescent screen. Each X-ray photon absorbed by the primary screen 15 is thus converted into a number of light photons close to 10,000, emitted by the luminescent screen 11.
Furthermore, the electronic optical system of the tube generally concentrates the output image on a far smaller format than that of the input image, typically a format equal to 1/10 to 1/5. This is accompanied by a major gain in luminance for this output image. The reduction of the image also means that the 1 mm details on the primary screen are reduced to about 1/10 mm on the luminescent screen, and that the image resolution required at the level of the luminescent screen is thus far greater than that detected at the primary screen.
The photon gain, and the gain in luminance provided by the reduction, make it possible, with radiological doses that can be borne by the patient, to obtain an output image that is luminous enough to be observed and recorded by means of a cinema camera or a television camera, in constituting radioscopic systems that work in real time.
In second and third generation light image intensifier tubes (image intensifiers in which the incident radiation is in the form of visible light and which therefore comprise no scintillator), there are known ways of adding an array of microchannels in order to further increase the electron gain. However, in X-ray image intensifier tubes such as those shown in FIG. 1, the photon gain is considered to be sufficient in practically every application, and it is generally not deemed to be necessary to increase it by adding a microchannel array, although assemblies such as these have already been proposed.
However, the use of a microchannel array in X-ray image intensifier tubes to replace the electronic optical system is considered to be likely to show great advantages such as, for example, great reduction in thickness, namely in the distance between the input window and the output port; uniform resolution on the entire image field (even for large-sized images); the possibility of making it far easier to obtain square or rectangular formats that are better suited to the formats of usual images or of television screens.
X-ray image intensifier tubes using a microchannel array instead of the electronic optical system are often called X-ray image intensifier tubes with dual proximity focusing. Tubes such as these are described notably in I.C.P. Millar et al., "Channel Electron Multiplier Plates in X-Ray Image Intensification", in Advances in Electronics and Electron Physics, Vol. 33, Academic Press, 1972. In the X-ray image intensifier tube described in this publication, the primary screen is plane. It is stretched in parallel to and at a small distance from the input face of the microchannel array, while the luminescent screen is placed in parallel to the output face of the array, and at a small distance from said array. To prevent the dispersal of the electrons, firstly between the photocathode and the input of the array and, secondly, between the output of the array and the luminescent screen, from causing deterioration in the resolution, very small distances, typically of less than 1 millimeter, have to be maintained between the electrodes.
FIG. 2 gives a schematic view of an X-ray image intensifier tube such as this, of a type similar to the one described in the above-mentioned publication.
As in the example of FIG. 1, the X-ray image intensifier tube 20 comprises a tube body 2 positioned about a longitudinal axis 3. The body 2 is closed at one end by an input window 4 and at the other end by an output port 5.
The incident X-rays penetrate the tube 20 by the input window 4 and then encounter a primary screen 21.
Unlike in the primary screen 15 of FIG. 1, the primary screen 21 of this version is plane. It has a scintillator support 22, a scintillator 23 and a photocathode 24 which may be of a same nature and which fulfil the same functions as the support, the scintillator and the photocathode shown in FIG. 1.
The electrons (not shown) emitted by the photocathode 24 are directed by an electrical field towards the input face 26 of a microchannel array 25. To this effect, a first biasing potential and a second biasing potential V1, V2 are applied respectively t the photocathode 24 and to the input face 26, the second potential V2 being more positive than the first potential V1.
The array of microchannel array 25 is an assembly of a multitude of small parallel channels or microchannels 27 separated by partitions 28, and assembled in the form of a rigid plate. Each primary electron (sent out by the photocathode) that penetrates a microchannel 27 is multiplied by a phenomenon of secondary emission in cascade on the walls of the microchannel, so that the electron current at the output of the array can be more than a thousand times greater than the electron current at input. The diameter d1 of the microchannels may range from 10 to 100 micrometers. The microchannels 27 are inclined with respect to the normal to the plane of the array so that electrons emitted by the photocathode 24 in parallel to this normal cannot emerge from a microchannel without having given rise to a phenomenon of secondary emission. In order to reduce the number of electrons that strike the input face 26 of the array 25 outside the microchannels, it is the usual practice to make a widened or flared portion 30 at the entrance to these channels and hence to reduce the thickness of the partition walls 28. The thickness E of the plate that forms the microchannel array 25 is typically between 1 and 5 mm. The electronic gain of the array may be adjusted over a wide range of values, for example between 1 and 5000, as a function of the voltage developed between the input face 26 and an output face 31 of this array 25, namely an output face 31 to which a third biasing potential V3 is applied.
The input face 26 and the output face 31 are each covered with a metallization layer, M1, M2 respectively (represented in FIG. 2 in dark lines) through which the potentials V2, V3 are distributed over the input and output faces. Naturally, these metallizations M1, M2 should not block the microchannels 27. It must be noted that it is common practice to deposit the metallization layers M1, M2 on the walls of the microchannels 27 at the ends of these microchannels, i.e. at the input and at the output of these microchannels. Generally, the metallization layers M1,M2 are deposited on the input and output faces 26, 31 of the microchannel array by a method of vacuum evaporation of a conductive material (such as, for example, chromium, nickel-chromium, Iconel etc.,) by Joule effect in using, most often, an electron gun to sublimate the metal to be evaporated.
This technique is a standard one. To limit the penetration of the metal into the channels 27, the evaporation is done at a glancing incidence ("slantwise").
Furthermore, the microchannel array is supported, during the evaporation, on a system of planet wheels which make it possible, by continuous rotation, to expose the surface of the array to the metal flow in every direction while, at the same time, preserving the glancing incidence. The penetration of the metal into the channels 27 is thus uniform, for each channel and for all the channels.
The electrons at output of the array of microchannels are accelerated and focused by an electrical field, on a luminescent screen (35) positioned so as to be facing the array, parallel to this array, and at a distance D of the order of 1 to 5 mm. The luminescent screen 35 has dimensions substantially equal to those of the primary screen. It locally emits a quantity of light proportional to the incident electron current, and it therefore restores a visible and intensified image of the X-ray image projected on the scintillator, through the input window of the tube. The luminescent screen 35 is a layer with a thickness of several micrometers, constituted by grains of luminophor material, and it may be deposited on the output port 5. The face of the luminescent screen 35, pointed towards the array 25 of microchannels, is coated with a very thin metal layer 36, made of aluminium for example. This metallization enables the electrical biasing of the screen (by the application of a fourth potential V4 that is more positive than the third potential V3) and acts as a reflector for the light reflected rearwards by this screen.
The primary screen 21 and the array 25 of microchannels are fixedly joined to the body 2 of the tube, for example by means of lugs 29 sealed to this body. To these lugs, there are furthermore applied the biasing potentials V1, V2, V3. The primary screen 21 and the slab 25 are thus fixed so as to be electrically insulated from each other while, at the same time, being separated by a relatively small distance D1 of the order of some tens of millimeters (it must be noted that, for greater clarity, the figures have not been drawn to scale).
A structure of a X-ray image intensifier tube such as this is difficult to manufacture, especially for large-sized images. It is indeed difficult to make a perfectly plane primary screen and to maintain it in a position parallel to the microchannel array, at a very small, uniform distance. However, this is necessary to limit the angular dispersal of the electrons (an effect which reduces the spatial resolution) and to obtain efficient resolution of the image on the entire field.
Another difficulty arises out of the fact that the scintillator 23 and its support 22 do not have the same expansion coefficients: they are both constituted by thin layers that tend to get deformed, and lead to a deformation of the photocathode and hence to a local modification of the distance between this photocathode and the microchannel array.
These difficulties are all the more pronounced as the dimension of the X-ray image intensifier tubes is great, whereas the applications envisaged for an X-ray image intensifier tube with a microchannel array (namely an X-ray image intensifier tube with dual proximity focusing) calls for large useful surface areas, typically with a diameter of over 15 cm, or equivalent surface areas in rectangular formats.